John, there are several reasons for a limited flash sync, and it may or may not matter for the type of photography you do.
First some basics: a flash photograph is really two exposures - one for ambient light and another for the flash exposure. Flash sync is when the sensor (or film emulsion) and the flash and ambient light can be recorded. Typically a camera's sync speed is determined by the how fast the camera's first and second curtains travel, leaving the sensor or film emulsion exposed to light.
Under "normal" circumstances a sync of 1/60s is fine for most indoor flash pics. Out door, you may want a faster sync speed of 1/125s or 1/250s. Typically this is the fastest sync speed most cameras can sync the exposure. This allows for the ambient light to be recorded as well as the flash burst Faster than this and the first and second shutters are a small slit travelling across the sensor/film emulsion area and the flash burst will not record evenly in the frame.
High speed sync is available on higher end camera bodies and speedlights. With high speed sync, the speedlight will actually fire a series of flash bursts to coincide with the shutter slit that exposes the sensor/film emulsion. Typically, however, the flash range will be shorter due to the need to charge the capacitor.
Normally, sync takes place with the shutter's first curtain, meaning that as the first curtain is open, the flash burst goes off, then the second curtain closes the shutter. Rear sync times the flash burst to go off before the second curtain closes. Slow sync removes the lower limit (typically 1/60s) and will sync with the first curtain. Slow Rear sync will again, remove the lower limit of the and sync the flash burst before the second curtain closes.
When would you use rear sync? Say following a car moving forward outside. If you sync at the first curtain (normal sync), the flash will illuminate the subject (car) at the beginning of the frame and the car would continue moving forward and the tail lights will be recorded through the car, giving the appearance that the car was in reverse. With rear sync, the shutter goes up, ambient light records the tail lights moving across the frame, flash burst illuminates the car at the end of the exposure and the second curtain ends the exposure, rendering the image how we typically perceive that to take place.